I sent an email to the vet yesterday and am waiting on a reply. I didn't try to make an appointment because I don't think they would have suggested anything else, and we were only onto day 4 of 10 of the antibiotics so they may have needed a bit of time to kick in.
And I think I made the right decision! She's perked up again, been fighting the meds, and is moving around much more, so I think she's starting to feel better. The next step would be an ultrasound, so will continue to keep an eye on her.
In slightly devastating news, I found out that our go-to vet, Lianne, is leaving the practice and I shed a few tears about it last night. I'm so grateful for all she's done to support my guineas over the years!